1. Field of the Invention
The present invention relates to a polishing cloth and a method of manufacturing a semiconductor device.
2. Description of the Related Art
It is known in the art that a polishing cloth is used in the manufacturing process of a semiconductor device in the cases where a semiconductor substrate, e.g., a semiconductor wafer, is mirror-finished by the chemical mechanical polishing treatment, where an insulating film is etched back for forming a buried insulating film in the semiconductor wafer (i.e., a buried element isolating region), and where a metal film is etched back for forming a buried wiring.
The polishing cloth known in the art is constructed to comprise a base body consisting of a hard polyurethane foam or a two-layer structure consisting of a hard polyurethane foam and a polyurethane unwoven fabric, and a surface layer of the base body having fine irregularities. The polishing cloth of the particular construction is used for polishing an insulating film deposited on the surface of a semiconductor wafer having, for example, a trench formed therein so as to form a buried insulating film (i.e., an element isolating region). To be more specific, the semiconductor wafer is held by a holder such that an insulating film, which is to be polished and formed on the semiconductor wafer, is allowed to face the polishing cloth. The semiconductor wafer having the insulating film formed thereon is pushed by the holder toward the polishing cloth under a desired load, and the holder and the polishing cloth are rotated in the same direction while supplying a polishing slurry containing abrasive grains from a supply pipe onto the polishing cloth so as to polish the insulating film formed on the semiconductor wafer.
In the polishing treatment described above, the abrasive grains contained in the polishing slurry and having a diameter of, for example, about 0.2 μm are loaded in the open cells, which generally have a diameter of 40 to 50 μm, of the polishing cloth so as to be dispersed uniformly between the polishing cloth and the insulating film formed on the semiconductor wafer. The abrasive grains are also held in the polishing cloth portion between the adjacent open cells of the polishing cloth. It follows that the insulating film formed on the semiconductor wafer is mechanically polished.
However, during the polishing treatment for a long time, the abrasive grains are accumulated in the open cells so as to increase the amount of the abrasive grains present in the polishing cloth portion between the adjacent open cells of the polishing cloth. In other words, the polishing force produced by the abrasive grains is increased. As a result, the polishing performance fluctuates such that the polishing rate is increased with time, compared with the polishing rate in the initial polishing stage.
It was customary in the past for the polishing cloth in which the polishing performance fluctuated as described above to be processed with a dressing apparatus for regeneration of the polishing cloth. The dressing apparatus noted above comprises a dressing tool of a construction wherein a large number of diamond particles are attached to a metallic base body by means of electrodeposition. However, it is necessary to apply the dressing treatment noted above every time the target object to be polished is subjected to a polishing treatment and, thus, the polishing operation is rendered troublesome. Also, it is possible for the surface of the target object to be polished to be scratched in the polishing stage by the diamond particles dropping from the dressing tool during the treatment with the dressing apparatus.
On the other hand, a polishing pad that makes it possible to obtain satisfactory polishing characteristics without employing a dressing treatment is disclosed in Japanese Patent Disclosure (Kokai) No. 2001-179607. The polishing pad disclosed in this patent document is formed of a resin, in which the amount of change in the center line average roughness, i.e., the Ra value, after the polishing of a single silicon wafer having an oxide film formed thereon is not larger than 0.2 μm based on the surface irregularity profile formed by the dressing treatment before the polishing stage. For example, the polishing pad noted above is formed of a resin prepared by dispersing polyvinyl pyrrolidone in a liquid phenolic resin or polymethyl methacrylate.
However, the patent document noted above does not refer to the specific materials in conjunction with the control of the Ra value of the polishing pad. In addition, the polishing pad disclosed in this patent document gives rise to the problem that the polishing rate is lowered.
In contrast, a polishing pad excellent in polishing characteristics such that damage such as scratches is not generated in the oxide film that is to be polished is disclosed in Japanese Patent Disclosure No. 2001-291685. The polishing pad disclosed in this patent document is prepared by dispersing fine elements having a high molecular weight such as rubber in an acrylic resin such as an acrylic copolymer.
However, open cells are present on the surface of the polishing pad disclosed in the patent document noted above, with the result that abrasive grains are accumulated in the open cells during the polishing treatment for a long time, giving rise to the problem that the polishing performance fluctuates.
Further, a polishing cloth capable of exhibiting stable polishing performance over a relatively long time without employing a dressing treatment is disclosed in Japanese Patent Disclosure No. 2002-190460. The polishing cloth disclosed in this patent document includes a polishing layer containing a high molecular weight material such as a silyl ester or a vinyl ether adduct of a carboxylic acid.